CT scanners known in the art typically produce a plurality of parallel, planar image slices. The slice planes are generally defined by the plane of revolution of an X-ray tube, which is mounted on an annular gantry, so as to revolve about a subject being imaged. The subject lies on a bed, which is translated through the gantry along a scan axis. An array of X-ray detectors on the opposite side of the subject from the X-ray tube receives radiation transmitted through the subject. Each of the detectors generates signals proportional to the attenuated X-ray flux incident thereon. These signals are pre-processed to produce attenuation data, which are filtered and back-projected to determine CT values, representing image data, at a plurality of points in each of the plurality of planar image slices. These slices, which are located at a corresponding plurality of mutually-spaced axial positions, are taken together to provide a three-dimensional image of all or a part of the subject's body.
Usually the scan axis is approximately parallel to the long axis of the subject's body and perpendicular to the plane of revolution of the tube, so that the image slices represent a series of axial cross-sections through the body. Such slices are known as "axial slices." Axial slices commonly have a relatively high image resolution, for example 1 mm or less, within the plane of each slice, but a poorer resolution in the axial dimension, for example about 5 mm. This axial resolution is known as the slice "thickness." Typically the spacing between one axial slice and the next is set to be approximately equal to the slice thickness.
In some cases, however, a physician examining the subject wishes to observe oblique. rather than axial, image slices, intersecting the body along planes that are angled with respect to the body's long axis. Several methods are known in the art for enabling such oblique slices to be viewed.
In some CT scanners, oblique image slice data are captured by angling the bed relative to the plane of revolution of the tube, by swiveling the bed and/or by tilting the tube's plane of revolution. The mechanisms required to implement and control the tilt and swivel capabilities, however, add substantially to the cost of the CT scanner. Furthermore, mechanical and other practical considerations limit the tilt and swivel angles to no more than about 30.degree., so that more oblique image slices, such as sagittal or coronal slices through the subject's body, cannot be directly produced.
Alternatively, an oblique image slice may be reconstructed by interpolation of the CT values in the original axial image slices. Such interpolative methods are well known in the art. as described, for example, in U.S. Pat. No. 4,674,046, to Ozeki et al., which is incorporated herein by reference. To view an oblique image slice according to this method, a user of a CT system, such as a physician, must generally operate the system first to acquire and display a full series of axial images, covering all or at least a substantial portion of the subject's body. Only afterwards may the user input to the system the position and orientation of the desired oblique slice.
Each oblique slice to be reconstructed in this manner intersects a number of the axial slices. For each of a plurality of points in the oblique slice, an interpolated CT value is found by taking a weighted sum of the CT values at a group of neighboring points in one or more of the original axial image slices. When using this method, typically only the CT values are stored in computer memory, while the attenuation data that were used to derive the CT values are deleted in order to conserve memory space.
Derivation of oblique slices by interpolation of axial images has several serious drawbacks. Interpolation of the CT values inevitably leads to a loss of resolution in the oblique image slice, since the axial resolution of the data is low. The interpolated oblique image slices generally appear to have poorer image quality, particularly higher noise level, and may exhibit artifacts, such as stair-step effects. Furthermore, the interpolation operation is computation-intensive and, consequently, time-consuming, and requires a very large computer memory.